Power reduction

ABSTRACT

There is disclosed a technique in which any peaks above a threshold level are reduced, but not clipped, such that the effects of such peaks is reduced. Although the implementation of the technique preferably includes a clipping step, it is performed on the front-end rather than as the last step in the technique, such that the output signal is not a clipped signal. Any noise introduced by the clipping step, so-called clipping noise, is preferably filtered out of the useful frequency band of the signal.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.09/634,750, filed on Aug. 9, 2000, which is incorporated herein byreference in its entirety, which claims Priority to United KingdomPatent Application 0015236.3, filed Jun. 21, 2000, and United KingdomPatent Application 0017726.1, filed Jul. 19, 2000.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a technique for reducing the maximalsignal amplitude and hence the crest factor of a signal, for example, amulti-carrier signal such as a DMT (discrete multi-tone) signal, andparticularly but not exclusively to such a signal generated by a modem.

2. Background Art

The amplitude distribution of a band-limited, over-sampled signal suchas a multi-carrier signal, generated by an ADSL (asymmetric digitalsubscriber line) modem, can be considered as Gaussian with a zero meanand a variance equal to the power of the transmitter signal. Theenvelope of such multi-carrier signals randomly present, with a smallprobability, peaks of a very high amplitude. The span (i.e. the peak topeak voltage) of signals driven by the line driver, which is thecomponent dedicated to transmit the power on the line, is important. Thepower consumption of the line driver is strongly related to the span ofthe voltage. In order to keep a reasonable level of power consumption, acertain amount of “clipping” of voltage peaks is desired. Such clippingreduces the voltage peaks of the signal.

However, any such clipping generates broadband noise, usually on thewhole bandwidth of the signal. This noise causes loss of data: in thetransmit direction by suddenly increasing the background noise; and inthe receive direction due to the leakage of the noise through thehybrid. This effect in the receive direction is mainly visible insystems utilising echo cancellers.

Consequently, too frequent clipping causes degradation of the overallbit error ratio of the transmission, resulting in data being lost. Thus,whilst a small amount of clipping is allowable and tolerable, theprobability of clipping occurring must be small enough to ensure goodperformance, i.e. no significant data loss.

U.S. Pat. No. 6,038,261 discloses a method for set-up of a signal inmulti-carrier modulation, including clipping of the signal amplitude. Afeedback loop is utilised to reinject a clipping noise from a previouspulse into a new pulse. The clipping noise is redistributed outside theuseful slip of the signal. This method ensures that the signal neverexceeds a predetermined threshold, since the output signal is obtaineddirectly from the output of a clipping circuit. However, the use of afeedback loop makes the behaviour of the technique unpredictable whenprocessing a sequence of pulses. Whilst the technique works in apredictable fashion when processing two successive peaks, feeding there-distributed noise of the first back to the second, with a sequence ofpeaks operation would be unstable. With a sequence of peaks it would bedifficult to predict the contribution made by successive peaks in thefeedback loop, and the system would be too unpredictable for reliableoperation.

It is an object of the present invention to provide a technique thatreduces the maximum signal amplitude of a multi-carrier signal whilstmaintaining the integrity of the signal. In particular the inventionaims to reduce the maximum signal amplitude whilst maintaining the biterror ratio of the original multi-carrier signal. Consequently theinvention reduces the crest factor of the signal.

SUMMARY OF THE INVENTION

In one aspect the present invention provides a method of processing aband-limited, over-sampled signal comprising: reducing the amplitude ofthose portions of the signal having peaks above a threshold value; andcontrolling the frequency position of the noise associated with thereduction of such peaks. Thereby the crest factor of the signal isreduced. The amplitude of those portions of the signal having peaksabove a threshold value are not limited to the threshold value. Whilstthey may be reduced to a value at or below the threshold value, they maybe reduced to a level which is above the threshold value.

The step of reducing the amplitude may comprise: clipping the signalrelative to the threshold; filtering the clipped signal; and combiningthe filtered clipped signal with the band-limited, over-sampled signal.The step of clipping the signal may comprise: limiting the amplitude ofthe signal to the threshold; and subtracting the limited signal from theband-limited, over-sampled signal.

The step of filtering the clipped signal may comprise creating a pulsehaving a pre-determined shape dependent upon the clipped samples.

The step of combining may comprise subtracting the filtered signal fromthe band-limited, over-sampled signal.

The step of combining may comprise delaying the band-limited,over-sampled signal by an amount corresponding to the time taken toimplement the clipping and filtering steps.

The step of moving the noise may comprise filtering. The step ofcontrolling the frequency position of the noise, may comprise moving thenoise outside the frequency band used by the signal.

The step of controlling the frequency position of the noise may comprisemoving the noise outside the frequency band used by a signal transmittedin the opposite direction. The signal may contribute an echo to thesignal transmitted in the opposite direction. Although this may resultin the clipping noise being present in the used band of the transmittedsignal, there may be circumstances where it is more desirable to ensurethat no clipping noise contributes to the echo.

In another aspect the present invention provides a method of processinga band-limited, over-sampled signal comprising: clipping the signal at agiven threshold; subtracting the clipped signal from the signal;filtering the subtracted signal to thereby control the frequencyposition of the clipping noise; delaying the signal; and subtracting thefiltered signal from the delayed signal, thereby reducing the amplitudeof those portions of the signal housing peaks above the threshold.

In a further aspect the invention provides a circuit for processing aband-limited, over-sampled signal, comprising: circuitry for reducingthe amplitude of those portions of the signal having peaks above athreshold value; and circuitry for controlling the frequency position ofthe noise associated with such peaks.

The circuitry for reducing may comprise: a limiter for clipping thesignal relative to the threshold; a filter for filtering the clippedsignal; and an arithmetic unit for combining the filtered clipped signalwith the signal. The arithmetic unit is preferably a subtractor.

The circuit may further include a delay circuit for providing a delayedversion of the signal to the arithmetic unit.

The circuitry for controlling the frequency position of the noisepreferably comprises a filter. The filter preferably controls thefrequency position of the noise to move the noise outside the usefulfrequency band of the signal.

The circuitry for controlling the frequency position of the noise maycontrol the noise to be positioned outside the frequency band used bythe signal. The circuitry for controlling the frequency position of thenoise may control the noise to be positioned outside the frequency bandused by a signal transmitted in the opposite direction. The signal maycontribute an echo to the signal transmitted in the opposite direction.

In a still further aspect the present invention provides a circuit forprocessing a band-limited, over-sampled signal comprising: a limiter forclipping the signal to a given threshold; a first subtractor forsubtracting the clipped signal from the multi-carrier signal, therebygenerating clipped samples; a filter for filtering the clipped samplesto thereby control the frequency position of the clipping noise; a delaycircuit for generating a delayed version of the multi-carrier signal;and a second subtractor for subtracting the filtered clipped samplesfrom the delayed multi-carrier signal, wherein the amplitude of thoseportions of the signal having peaks above the threshold is reduced.

BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES

The invention will now be described with regard to a non-limitingexample with reference to the accompanying drawings in which:

FIG. 1 illustrates a block diagram of a circuit for implementing thetechnique of the present invention; and

FIGS. 2 to 6 illustrate waveforms at various points in the circuit ofFIG. 1 in an illustrative example.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The invention is described herein with reference to a particular exampleconcerning the processing of a signal generated by an ADSL (asymmetricdigital subscriber line) modem. Such a modem generates a DMT (discretemulti-tone) signal carrying multiple carrier frequencies. Such signalsare band-limited, over-sample signals. However, the invention is notlimited to the processing of such a signal, and may be more broadlyapplied to the processing of any band-limited, over-sampled signal. Theinvention is not limited in its applicability to multi-carrier signals,but may also apply to single carrier signals. The following descriptionuses the example of a multi-carrier signal for illustrative purposesonly.

As discussed in the introduction hereinabove, as a result of thecombining of multiple carriers, the signal at the output of the modemcontains random peaks which are above a desired threshold. The level ofthe desired threshold is user or implementation dependent.

Referring to FIG. 1, there is illustrated a block diagram of a digitalcircuit suitable for processing the multi-carrier signal at the outputof the modem in accordance with a preferred embodiment of the presentinvention. The thus processed signal is then suitable for inputting to aline driver.

The digital circuit, generally designated by reference numeral 12,comprises a clipper or limiter 2, a filter 6, a delay unit 10, and twoarithmetic or subtractor units 4 and 8.

The input to the digital circuit 12 on line 14 is, in this illustrativeembodiment, the multi-carrier DMT signal comprising samples from theADSL modem. The samples from the ADSL modem are over-sampled in order toobtain a band-limited signal. The samples from the ADSL modem on line 14form an input to the limiter 2. Referring to FIG. 2, there isillustrated an example of the samples on line 14 forming an input to thedigital circuit.

The limiter 2 clips the samples from the ADSL modem on line 14 at agiven threshold and provides clipped samples on line 16.

The threshold applied in the limiter 2 is determined based on thetolerable degradation of the bit error ratio, and will be user orimplementation dependent. In general the lower the required bit errorratio, the higher the required threshold. The principles andimplementation of such clipping are well-known in the art.

In the present example, it is assumed that the threshold value is alevel V_(TH). The limiter or clipper 2 thus generates a sequence ofpulses including four pulses corresponding to the four pulses above thethreshold value V_(TH) as shown in FIG. 3. As can be seen in FIG. 3, theamplitude of these four pulses is limited at the output of the limiteror clipper 2 to the threshold value V_(TH).

The clipped samples on line 16 are provided to the arithmetic unit 4together with the samples from the ADSL modem on line 14. The arithmeticunit 4 subtracts the clipped samples on line 16 from the samples fromthe ADSL modem on line 14, and a sequence of clipped samples is thusgenerated on line 18, as shown in FIG. 4.

Referring to FIG. 4, it can be seen that the clipped samples indicatefour samples corresponding to the four original samples above thethreshold value V_(TH). The amplitude of these samples corresponds totheir amplitude above the threshold value V_(TH).

Thus the subtraction performed by the subtractor 4 extracts the part ofthe samples of the incoming signals which are above the threshold.

The sequence of clipped samples on line 18 are then provided as an inputto the filter 6. The filter 6 filters the clipped samples on line 18 toshape the noise generated by the clipping operation. The filter computesa pulse from the clipped samples which has a spectrum outside the signalbandwidth of the useful signal. Thus, in this preferred embodiment, thefilter outputs a sequence of samples which have a spectrum outside theused band of the multi-carrier ADSL signal generated by the modem.

This type of processing, where the samples are moved out of band, isknown as soft-clipping. By contrast, hard-clipping would result in thesamples (and thus the noise) remaining in band. Soft-clipping techniquesusing a filter are well-known to one skilled in the art. Similarly theuse of filters generally to control the frequency position of a signalis well-known.

In this preferred embodiment the filtering is carried out using a FiniteImpulse Response Filter.

Although in this preferred embodiment the filter is utilised to ensurethe noise is out of bound, in general it would be possible to derivefrom the clipped samples a pulse with a “good” spectrum. However,without shifting the noise outside the used frequency band goodperformance is not achieved in the illustrative embodiment describedherein.

In general terms, the filter controls the frequency position of thenoise associated with the peaks above the threshold value. In certainapplications, it may be advantageous to control the frequency positionwithin the useful band of the band-limited, over-sampled signal.

In one example of such a scenario, the noise associated with theclipping of the peaks above the threshold is controlled so as to beoutside the useful band of the signal received in the modem. As iswell-known in the art, the signal received in the modem is susceptibleto echo effects due to the signal transmitted in the same modem, theecho effect being a characteristic of a hybrid used to transmit andreceived signals on the communication channel. By ensuring that theclipping noise is moved outside the useful band of any signal receivedin the hybrid of the modem, then any clipping noise leakage in the echois reduced. Although this may result in the clipping noise being presentin the used band of the transmitted signal, there may be circumstanceswhere it is more desirable to ensure that no clipping noise contributesto the echo.

In the present illustrative embodiment, and in most applications, it ispreferable to move this noise outside the useful band.

Thus, in the illustrative embodiment the filter provides a modifiedversion of the clipped samples in the incoming signal in anotherfrequency spectrum. The shape of the filtered clipped samples is shownin FIG. 5.

The sequence of filtered clipped samples generated by the filter 6 areprovided on line 20 to the arithmetic unit 8. The arithmetic unit 8 alsoreceives on line 24 a delayed version of the input signal on line 14.The delayed signal is provided by the delay unit 10. The delay isprovided to compensate for the computation time in the limiter 2 and thefilter 6, such that the timing of the delayed version (or replica) ofthe input signal coincides with the pulses at the output of the filter.The filtered clipped samples on line 20 are subtracted from the delayedversion of the input signal on line 24 in the subtractor 8. Thissubtraction removes the pulse, generated from the clipped samples, fromthe incoming signal.

The waveform resulting from this subtraction is illustrated in FIG. 6.As can be seen from FIG. 6, although the signal still has peaks abovethe threshold value V_(TH), the peaks of the signal which originallyexceeded V_(TH) are reduced with respect to the original signal. Thepeak voltage and the power consumption of the signal provided to theline driver on line 22 are thus decreased. Although FIG. 6 illustratesthat none of the peaks are actually reduced below V_(TH), those peakswhich are initially only a certain level above V_(TH) may be reduced toa level at or below V_(TH).

The crest factor of the final signal output by the digital circuit 12 online 22 depends on three factors:

-   -   1. The maximal crest factor of the input signal on line 14;    -   2. The threshold value used in the limiter 2; and    -   3. The way in which the pulse is derived from the clipped        samples in the filter 6.

There is an obvious relationship between the first two factors and thefinal crest factor: the higher the maximal input, the higher the maximaloutput; and the higher the threshold value, the higher the maximaloutput.

As regards the filter, optimisation of the design to adjust the finalmaximal crest factor will be dependent upon the design of the filteritself. The design of the filter is outside the scope of the presentinvention, but will be readily realised by one skilled in the art.

Thus the present invention provides a technique in which any peaks abovea threshold level are reduced, but not clipped, such that the effects ofsuch peaks is reduced. Although the implementation of the techniquepreferably includes a clipping step, it is performed on the front-endrather than as the last step in the technique, such that the outputsignal is not a clipped signal. Any noise introduced by the clippingstep, so-called clipping noise, is preferably filtered out of the usefulfrequency band of the signal.

Although the invention is described herein with respect to processingsamples in the digital domain, the principles of the invention applyalso to the analogue domain.

1. A method of processing a band-limited, over-sampled signalcomprising: reducing the amplitude of those portions of the signalhaving peaks above a threshold value; and controlling the frequencyposition of the noise associated with the reduction of such peaks. 2.The method of claim 1 wherein the step of reducing the amplitudecomprises: clipping the signal relative to the threshold; filtering theclipped signal; and combining the filtered clipped signal with theband-limited, over-sampled signal.
 3. The method of claim 2 wherein thestep of clipping the signal comprises: limiting the amplitude of thesignal to the threshold; and subtracting the limited signal from theband-limited, over-sampled signal.
 4. The method of claim 2 wherein thestep of filtering the clipped signal comprises creating a pulse having apre-determined shape dependent upon the clipped samples.
 5. The methodof claim 2 wherein the step of combining comprises subtracting thefiltered signal from the band-limited, over-sampled signal.
 6. Themethod of claim 2 wherein the step of combining comprises delaying theband-limited, over-sampled signal by an amount corresponding to the timetaken to implement the clipping and filtering steps.
 7. The method ofclaim 1 wherein the step of moving the noise comprises filtering.
 8. Themethod of claim 1 wherein the step of controlling the frequency positionof the noise comprises moving the noise outside the frequency band usedby the signal.
 9. The method of claim 1 wherein the step of controllingthe frequency position of the noise comprises moving the noise outsidethe frequency band used by a signal transmitted in the oppositedirection.
 10. The method of claim 9 wherein the signal contributes anecho to the signal transmitted in the opposite direction.
 11. A methodof processing a band-limited, over-sampled signal comprising: clippingthe signal at a given threshold; subtracting the clipped signal from thesignal; filtering the subtracted signal to thereby control the frequencyposition of the clipping noise; delaying the signal; and subtracting thefiltered signal from the delayed signal, thereby reducing the amplitudeof those portions of the signal having peaks above the threshold.
 12. Acircuit for processing a band-limited, over-sampled signal, comprising:circuitry for reducing the amplitude of those portions of the signalhaving peaks above a threshold value; and circuitry for controlling thefrequency position of the noise associated with the reduction of suchpeaks.
 13. The circuit of claim 12 wherein the circuitry for reducingcomprises: a limiter for clipping the signal relative to the threshold;a filter for filtering the clipped signal; and an arithmetic unit forcombining the filtered clipped signal with the signal.
 14. The circuitof claim 13 wherein the arithmetic unit is a subtractor.
 15. The circuitof claim 12 further including a delay circuit for providing a delayedversion of the signal to the arithmetic unit.
 16. The circuit of claim12 wherein the circuitry for controlling the frequency position of thenoise comprises a filter.
 17. The circuit of claim 12 wherein thecircuitry for controlling the frequency position of the noise controlsthe noise to be positioned outside the frequency band used by thesignal.
 18. The circuit of claim 12 wherein the circuitry forcontrolling the frequency position of the noise controls the noise to bepositioned outside the frequency band used by a signal transmitted inthe opposite direction.
 19. The circuit of claim 18 wherein the signalcontributes an echo to the signal transmitted in the opposite direction.